1. Field of the Invention
This invention relates to a method of forming subterranean water barrier and a plow for use in effecting the method.
2. Prior Art
As the population of the world today continues to grow, the problem of a food crisis becomes more pressing. One solution to this problem which has increasingly been focused is to improve the deserts occupying one third of land of the earth in order to make it suitable for agricultural, grazing and foresting purposes.
A technique of improving the desert soil has already been known in early 1900's. However, a technique of forming an economical sub-surface water barrier which provides an improved water retention property for the improvement of the soil, is relatively new and has been developed from about 1966. Prior art methods of forming a subterranean water barrier has been described in U.S. Pat. Nos. 668,362, 3,276,208, 3,405,528, 3,405,529, and 3,398,542, for instance. These prior art efforts generally comprise the steps of forming a translating cavity by moving a plow in the underground, spraying asphalt in the liquid form onto a bottom surface of the cavity which is formed by the font edge of the plow in order to spread the asphalt freely, and covering the spread asphalt with soil to thereby form an asphalt film. The conventional efforts have however suffered from various drawbacks mentioned below.
The major problem experienced with the asphalt film is the formation of apertures therein which, as will be described later, greatly reduce a water retention property enrichment of which is a principal aim that the subterranean water barrier is to accomplish. Even those apertures which have a relatively small diameter allow water leakage which is more or less the same in amount as in the case of those of a large diameter.
The asphalt in the liquid form sprayed on the soil surface and exposed to air tends to be separated just as water on an oiled surface is separated due to surface tension to form discontinuous water films, with the result that the asphalt is difficult to be spread uniformly even if the liquid asphalt layer is increased in thickness considerably. One major cause for aperture formation in the course of barrier preparation is the separation of asphalt due to its surface tension when the asphalt is exposed to air as it is sprayed onto the bottom surface of a cavity formed by the plow. Furthermore, the surface of asphalt is waved or disturbed by an asphalt flow injected from a nozzle and by covering soil, so that the asphalt layer makes a complicated movement, tending to permit of the mixing of air which is liable to cause apertures to be formed in the asphalt layer. Additionally, the asphalt cannot be covered uniformly with soil, so that the asphalt layer will have a combination of thinned portions, thickened portions and severed portions which are formed by soil that directly covers the bottom surface of the cavity.
Moreover, the asphalt tends to be mixed with soil falling at a rapid speed to thereby form a fragile asphalt film which can be easily broken by the deformation of soil thereabove that is caused as vehicles pass on the soil surface.
One proposal to prevent the aperture formation would be to supply a great amount of asphalt into the plow. However, a large supply of asphalt further causes the disturbance of the asphalt layer by the action of covering soil. The disturbance of the layer extends over a portion where soil falls toward a portion rearward of the plow where the disturbed asphalt layer is covered with soil or it covers soil, with the result that the formed layer has a multiplicity of wrinkles which consume an excessive supply of asphalt. Thus, this proposal only wastes asphalt being supplied. With the prior art asphalt film formation process, the above deficiencies cannot be overcome due primarily to the liquid asphalt being exposed to air and hence an hydrodynamically complicated movement of the liquid asphalt. An attempt has been made to solve the deficiencies by producing a gaseous ammonia atmosphere in the cavity and introducing asphalt emulsion into the cavity. However, due to the above-mentioned characteristic of asphalt, that is, easy separation because of its surface tension, it is fatally disadvantageous to leave the sprayed asphalt as it is until its surface is covered with soil.
Another proposal to prevent the aperture formation would be to make a distance between adjacent spray nozzles short, to make the nozzles to be positioned high above the bottom surface of the cavity in order to overlap the sprayed films and also to increase the spray angle which can be obtained by increasing the nozzle pressure, and by reducing the diameter of the orifice of the nozzle. However increasing the nozzle pressure damages the bottom surface of the cavity where the asphalt is mixed with a greater amount of sand, so that a fragile asphalt film is formed and the film tends to be increased in thickness, resulting in larger consumption of asphalt. Thus, the thickness of the asphalt layer right beneath the nozzle is prone to be greater than that of other portion of the layer, the asphalt film being unequal in thickness both transversely and longitudinally thereof. The reduced diameter of the nozzle orifice causes the orifice to be choked. The choked nozzle in the injection type then contributes to much more defects in the asphalt film. To eliminate this problem, it is necessary that the nozzle could be positioned high above the bottom surface of the cavity under a low pressure in order to overlap the sprayed film, and that a portion where the asphalt layer is covered with soil could be spaced apart from the nozzle a larger distance. This effort however makes the plow structure bulky and thus results in increased resistance to the advance of the plow in the underground, which increased resistance then causes a greater power loss on the tractor.
Moreover, the injection type method meets with only partial success because it is suitable merely for the formation of a horizontal film, and unsuitable for the formation of an inclined film in which case asphalt tends to flow down by gravity along the inclined surface of the cavity.